1. No Toad
Generally, can see most mainland natl parks with a small RV. You should be able to get to just about every natl park via driving a small RV (except island ones and several Alaska ones) and drive to.and stay in campgrounds in the natl parks.
There are some parts of various natl parks where a smaller vehicle would be better or where even a small RV would be inadvisable BUT you have options of public tours (natl parks are trying to reduce traffic in parks so may have free shuttles) or rent a small vehicle or 4x4 if necessary for that park if absolutely must see something.
Zion shuts down the main road to all traffic during peak season except for their shuttles and bicycles (and hotel traffic). Yosemite has free shuttle bus in valley and (double check) a small RV should make it to the Toulm Meadows (tour buses do but windy road and tunnels). Grand Canyon has shuttles.
Issues are height restrictions (usually tunnels or bridges), width, and dirt roads. For example, Zion has a tunnel which requires driving down the middle, Glacier has a windy mountain road to the sun, etc. Death Valley has certain sites accessible via dirt roads only.
Can research beforehand by checking each park's website, getting Natl Geo Natl Park Guide, and getting info at each park's Visitor Center (advisable for info on road restrictions, closures, hiking areas closed off for bear activity, etc.).
The main attractions of most natl parks are usually easily accessible by a small RV. If you prefer more remote areas, then check with each park for that remote area and then decide whether worth it.
Note: Bicycles are a nice choice for many natl parks. Can often rent in the parks as well.
2. Planning
Check RV caravan tour catalogs for ideas re itinerary for natl parks (e.g., the natl park loop in the western states). Helpful for timing. The Natl Geo Natl Parks Guide is helpful to decide how many days to stay in each park and what to do for number of days. For example:
http://adventurecaravans.com/tour/grand-circle-the-great-western-national-parks-tour/
Also, trip routing:
http://www.randalolson.com/2016/07/30/the-optimal-u-s-national-parks-centennial-road-trip/
Should reserve campground sites way in advance as natl parks campground sites fill up way in advance. Look for nearby alternatives, including Allstays.com (app), PassportAmerica, etc.
Natl Parks vary as to available hookups in campgrounds
3. Pass
Get Senior (best deal ever) or Annual Pass.